Prior to the present invention, various one and two-package moisture curable room temperature vulcanizable (RTV) compositions were available based on the use of a silanol-terminated polydiorganosiloxane having the formula, ##STR1## where R is a C.sub.(1-13) monovalent substituted or unsubstituted hydrocarbon radical, which is preferably methyl, or a mixture of a major amount of methyl and a minor amount of phenyl, cyanoethyl, trifluoropropyl, vinyl, and mixtures thereof and n is an integer having a value of from about 50 to about 2500, with a cross-linking silane having hydrolyzable radicals attached to silicon.
Ceyzeriat, U.S. Pat. No. 3,133,891 and Bruner, U.S. Pat. No. 3,035,016, are based on the use of methyltriacetoxysilane with a silanol-terminated polydimethylsiloxane under substantially anhydrous conditions. Although the one-package compositions of Bruner or Ceyzeriat, upon exposure to atmospheric moisture, provide satisfactory one-package room temperature vulcanizable organopolysiloxane compositions exhibiting satisfactory tack-free time, for example, 30 minutes or less after an extended shelf period, the acetic acid by-product is corrosive and has a disagreeable odor.
Other variations of one-package acyloxy acid generating RTV's are shown by Kulpa, U.S. Pat. No. 3,296,161, Goossens, U.S. Pat. No. 3,296,195 and Beers, U.S. Pat. No. 3,438,930, assigned to the same assignee as the present invention. Additional one-package acyloxy acid generating RTV compositions are shown by Schulz et al, U.S. Pat. No. 3,647,917 and Nitzsche et al U.S. Pat. No. 3,886,118.
An improved, low odor, substantially non-corrosive one-package RTV composition is shown by Beers, U.S. Pat. No. 4,257,932, assigned to the same assignee as the present invention. Beers achieves a reduction in odor and corrosive properties by utilizing as a cross-linking silane, a less volatile material such as methyltris-(2-ethylhexanoxy)silane.
A non-corrosive two package moisture curable organopolysiloxane composition free of carboxylic acid generating groups is shown by Nitzsche et al, U.S. Pat. No. 3,127,363 which is based on the use of a polyalkoxysilane, or polysilicate cross linking agent, in place of methyltriacetoxysilane. The ingredients of the two package noncorrosive composition of Nitzsche et al, are mixed under atmospheric conditions and the resulting composition must be used soon after the ingredients are mixed because the resulting blend has a short shelf life. Although the mixture of Nitzsche et al, which is typically polyalkoxysilane, silanol-terminated polydiorganosiloxane and tin soap catalyst, provides upon mixing, a fast curing non-corrosive room temperature vulcanizable composition, the Nitzsche et al mixture does not have the extended shelf life advantage of the one package system which is required for various commercial uses and therefore is excluded from a variety of applications.
Nitzsche et al, U.S. Pat. No. 3,065,194, teaches that a mixture of an endblocked dimethylsiloxane polymer, such as hydroxy and alkoxy endblocked, inert filler, ethylorthosilicate and dibutyltindilaurate can be vulcanized upon contact with water, after a 14 day shelf period at room temperature. However, the various ingredients of the mixture have to be vigorously dried by heating for 1 hour at 200.degree. C., and the RTV, after a relatively short shelf period, has to be drenched with water.
Improved results toward combining the advantages of a non-corrosive acid-free polyalkoxysilane cross-linking agent with a silanol-terminated polydiorganosiloxane as a one-package system are shown by Weyenberg, U.S. Pat. No. 3,334,067, Cooper et al, U.S. Pat. No. 3,542,901 and by Smith et al U.S. Pat. Nos. 3,689,454, and 3,779,986, the last two being assigned to the same assignee as the present invention, utilizing a titanium chelate catalyst in place of a tin catalyst. However, after room temperature vulcanizable one-package systems based on a titanium chelate catalyst were allowed to age for a period of 5 hours or more, it was found that the tack-free time of the aged RTV was considerably longer than the tack-free time of the same mixture after it was initially mixed and immediately exposed to atmospheric moisture.
As shown by Brown et al U.S. Pat. No. 3,122,522, a platinum catalyst is used to prepare an alkoxy terminated silalkylenepolysiloxane polymer. However, this method of synthesizing the base polymer requires an expensive hydrosilylation procedure. Additional efforts to achieve a desirable non-corrosive, substantially odor-free stable one-package RTV based on the use of polyalkoxy organopolysiloxane in a more economic manner are shown by Brown et al, U.S. Pat. No. 3,161,614 or U.S. R. 29760. Brown et al employed a polyalkoxy end blocked polysiloxane which was based on the use of a mineral acid generating polyalkoxyhalosilane, and a curing catalyst. However, these compositions were found to be unusable because they failed to cure in contact with a tin catalyst, even in the presence of moisture.
As utilized hereinafter, the term "stable" as applied to the one package alkoxy-terminated organopolysiloxane RTV's of the present invention means a moisture curable mixture capable of remaining substantially unchanged while excluded from atmospheric moisture and which cures to a tack-free elastomer after an extended shelf period. In addition, a stable RTV also means that the tack-free time exhibited by freshly mixed RTV ingredients under atmospheric conditions will be substantially the same as that exhibited by the same mixture of ingredients exposed to atmospheric moisture after having been held in a moisture resistant and moisture-free container for an extended shelf period at ambient conditions, or an equivalent period based on accelerated aging at an elevated temperature.
In copending application Ser. No. 277,524, filed June 26, 1981, White et al, for One Package, Stable, Moisture Curable, Polyalkoxy-terminated Organopolysiloxane Compositions and Method for Making, assigned to the same assigned as the present invention, room temperature vulcanizable compositions are shown based on the use of a polyalkoxy terminated polydiorganosiloxane in combination with certain silane scavengers for materials having chemically combined hydroxy radicals.
Although valuable, substantially acid-free products have been obtained with the use of such silane scavengers which also can function as stabilizer in the room temperature vulcanizable organopolysiloxane compositions of Ser. No. 277,524, it has been found that the use of such silane scavengers can result in the production of undesirable by-products during the cure of the RTV composition. The aforementioned undesirable by-products, such as amides, amines, carbamates, oximes, can be generated which can interfere with the utility of the RTV composition. In addition, in view of the special procedures required to synthesize such silane scavengers, and the cost of materials used in such synthesis, the overall cost of the RTV composition can be significantly enhanced.
The present invention is based on the discovery that stable, substantially acid-free, one package, moisture curable RTV compositions can be made by utilizing a silicon-free scavenger, or organic scavenger, in place of the above described silane scavenger. Accordingly, these organic scavengers, as defined hereinafter, can be advantageously used in combination with alkoxy terminated polydiorganosiloxane of the formula, ##STR2## where R and n are as previously defined, R.sup.1 is a C.sub.(1-8) aliphatic organic radical selected from alkyl radicals, alkylether radicals, alkylester radicals, alkylketone radicals and alkylcyano or a C.sub.(7-13) aralkyl radical, R.sup.2 is a C.sub.(1-13) monovalent organic radical selected from R radicals as previously defined, and a is a whole number equal to 0 to 2 inclusive. It has been found that there can be utilized from about 0.1 to about 10 parts of the silicon-free scavenger or organic scavenger of the present invention per 100 parts of the alkoxy-terminated polydiorganosiloxane of formula (2) or the silanol terminated polydiorganosiloxane of formula (1). Preferably, 1 or 2 parts per 100 parts of organopolysiloxane polymer can be used.
In addition, in certain instances, organic amines can function as curing accelerators in combination with the above described organic scavengers. Furthermore, there can be utilized in the room temperature vulcanizable compositions of the present invention cross-linking polyalkoxysilane having the formula, ##STR3## where R.sup.1 is as previously defined, R.sup.2 is a C.sub.(1-13) monovalent organic radical selected from R radicals and b is a whole number equal to 0 or 1.
As a result of the substitution of the organic scavenger for the silane scavenger as previously discussed, the RTV compositions of the present invention can be made at substantially reduced costs. Based on experience with the use of compatible organic materials with silicones, those skilled in the art would anticipate that the paintability of the surfaces of tack-free cured products obtained from RTV compositions of the present invention would be enhanced as compared to cured products derived from prior art RTV compositions.